Potassium channels are involved in a number of physiological processes, including regulation of heartbeat, dilation of arteries, release of insulin, excitability of nerve cells, and regulation of renal electrolyte transport. Potassium channels are thus found in a wide variety of animal cells such as nervous, muscular, glandular, immune, reproductive, and epithelial tissue. These channels allow the flow of potassium in and/or out of the cell under certain conditions. For example, the outward flow of potassium ions upon opening of these channels makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell. These channels are regulated, e.g., by calcium sensitivity, voltage-gating, second messengers, extracellular ligands, and ATP-sensitivity.
Potassium channels are made by alpha subunits that fall into 8 families, based on predicted structural and functional similarities (Wei et al., Neuropharmacology 35(7):805–829 (1997)). Three of these families (Kv, Eag-related, and KQT, now referred to as KCNQ) share a common motif of six transmembrane domains and are primarily gated by voltage. Two other families, CNG and SK/IK, also contain this motif but are gated by cyclic nucleotides and calcium, respectively. The three other families of potassium channel alpha subunits have distinct patterns of transmembrane domains. Slo family potassium channels (also known as BK channels) have seven transmembrane domains (Meera et al., Proc. Natl. Acad. Sci. U.S.A. 94(25):14066–71 (1997)) and are gated by both voltage and calcium or pH (Schreiber et al., J. Biol. Chem. 273:3509–16 (1998)). Another family, the inward rectifier potassium channels (Kir), belong to a structural family containing 2 transmembrane domains (see, e.g., Lagrutta et al., Jpn. Heart. J. 37:651–660 1996)), and an eighth functionally diverse family (TP, or “two-pore”) contains 2 tandem repeats of this inward rectifier motif.
Potassium channels are typically formed by four alpha subunits, and can be homomeric (made of identical alpha subunits) or heteromeric (made of two or more distinct types of alpha subunits). In addition, potassium channels have often been found to contain additional, structurally distinct auxiliary, or beta, subunits (e.g., Kv, Slo, and KCNQ potassium channel families). These beta subunits do not form potassium channels themselves, but instead they act as auxiliary subunits to modify the functional properties of channels formed by alpha subunits. For example, the Kv beta subunits are cytoplasmic and are known to increase the surface expression of Kv channels and/or modify inactivation kinetics of the channel (Heinemann et al., J. Physiol. 493:625–633 (1996); Shi et al., Neuron 16(4):843–852 (1996)). In another example, the KCNQ family beta subunit, minK, primarily changes activation kinetics (Sanguinetti et al., Nature 384:80–83 (1996)).
The KCNQ family of potassium channels was first identified in humans on the basis of inherited mutations that cause the Long QT syndrome (Wang et al., Nat. Genet. 12:17–23 (1996)). The mutations were found in a potassium channel, KVLQT1, now known as KCNQ1, that was structurally distinct from previously cloned voltage-gated potassium channels. More recently, it has been discovered that KCNQ1 represents a larger family of structurally similar voltage-gated potassium channels. Three more members of this novel voltage-gated potassium channel family, KCNQ2, KCNQ3, and KCNQ4, have been cloned from humans (Charlier et al., Nat. Genet. 18:53–55; Biervert et al., Science 279:403–406 (1998); Singh et al., Nat. Genet. 18:25–29 (1998); Yang et al., J. Biol. Chem. 273:19419–19423 (1998); and Kubisch et al., Cell 96:437–446 (1999)). Mutations in each member of the KCNQ gene family have been linked to inherited human disease. For example, KCNQ1 has been linked to the Long QT syndrome, as described above. KCNQ2 and KCNQ3 have been linked to certain forms of epilepsy (Charlier et al., Nat. Genet. 18:53–55; Biervert et al., Science 279:403–406 (1998); and Singh et al., Nat. Genet. 18:25–29 (1998). KCNQ4 has been linked to deafness (Kubisch et al., Cell 96:437–446 (1999)).
KCNQ family genes are typically composed of four alpha subunits from a KCNQ family member and can be homomeric or heteromeric (Yang et al., J. Biol. Chem. 273:19419–19423 (1998); and Kubisch et al., Cell 96:437–446 (1999)). They are found in a variety of tissues and cell types, and contribute to such processes as neuronal excitability and integration, cardiac pacemaking and muscle contraction (Wang et al., Nat. Genet. 12:17–23 (1996); Charlier et al., Nat. Genet. 18:53–55; Biervert et al., Science 279:403–406 (1998); Singh et al., Nat. Genet. 18:25–29 (1998); Yang et al., J. Biol. Chem. 273:19419–19423 (1998); and Kubisch et al., Cell 96:437–446 (1999)). In particular, KCNQ family members contribute to M-currents, which are key to controlling neuronal excitability (Wang et al., Science 282:1890–1893 (1998)).